In recent years, in the automotive field, to lighten the weight of car bodies and improve safety in impact, the utilization of high strength steel sheets increases. The strength of the steel sheets used has also increasingly risen. In the process of assembly of the car bodies made of such high strength steel, attachment of parts, etc., spot welding using resistance welding has mainly been used.
The bonding strength of a spot welded part is evaluated by the tensile shear strength (TSS) measured by applying a tensile load in the shear direction and by the cross tensile strength (CTS) measured by applying a tensile load in the peel-off direction. It is known that this TSS and CTS change by the relations such as shown in FIG. 16 together with the increase of the tensile strength of the steel sheet. That is, the TSS is improved along with the rise of the tensile strength of the steel sheets, while the CTS becomes saturated at a tensile strength of the steel sheets of around 590 MPa and conversely falls along with the tensile strength of the steel sheets becoming higher than that tensile strength.
When spot welding high strength steel sheets with a tensile strength of over 590 MPa, a drop in the toughness and ductility of the nuggets arises along with a rise in the hardenability of the base metal steel sheet s. Further, when applying a peel-off load such as CTS in spot welding, the higher the strength of the steel sheets, the easier it is for the stress to concentrate at the ends of the weld metal (nuggets). Furthermore, when the pressing force is high or when scattering occurs, the thickness of the steel sheets at a spot welded part becomes thinner compared with the surroundings and the stress level ends up rising more compared with the applied load. Due to these, in spot welding, when the strength of the steel sheets is high, the CTS of the weld zone falls.
As opposed to this, in arc spot welding, it is possible to weld through the steel sheets and form an excess buildup part at the weld metal, so it is possible to obtain a higher strength of joint than the utilization of spot welding.
PLT 1 discloses an arc spot welded joint obtained by making the overlaid high strength steel sheets melt to the back surface side, forming weld beads so as to stick out from the surfaces of the steel sheets, and controlling the bead diameters to suitable ranges in relation to the thicknesses of the steel sheets so as to obtain both high CTS and TSS and realize a welded joint excellent in strength of joint.
However, the weld bead shapes change along with the chemical ingredients of the steel material, thickness of the steel material, etc. Furthermore, the weld bead of the back surface droops down due to the effect of gravity, so it is difficult to stably control the weld bead shape.
As the means for suppressing drooping of the back surface bead, PLT 2 discloses the method of using backing plate. PLT 2 proposes to prevent melt fusion of the backing plate and weld metal by the method of providing a recess at the front surface part of the backing plate to suppress the generation of an arc discharge to the backing plate.
FIG. 1 of PLT 2 describes that a weld bead of the same size as the recess of the backing plate is formed at the back side of the overlaid steel sheet. However, the inventors tried using the backing plate disclosed in PLT 2 to control the back surface bead shape at the time of arc spot welding, but it was not possible to obtain a weld bead diameter or excess metal height such as disclosed in PLT 1.
FIG. 2 of PLT 3 discloses an apparatus with a recessed gas supply groove provided with a width able to form the atmosphere at the time of arc welding, discharge the resin gas produced when butting and welding two vibration-damping steel sheets, and cool the back surface near the weld zone of the two vibration-damping steel sheets so as to be able to cool the vicinity of the end edges of the weld zone of the two vibration-damping steel sheets.
Further, FIG. 3 etc. of PLT 4 discloses an apparatus placing two steel sheets overlaid on a receiving table and performing arc spot welding.
However, neither PLT 3 nor 4 discloses a specific method for control of the buildup height of a weld bead or rest of the shape of a weld bead. For this reason, it is not self evident whether the above-mentioned apparatus has the functions enabling formation of a bead shape necessary for raising the strength of the weld zone.